Composite particle dispersions

ABSTRACT

A composite particle dispersion is prepared by mixing an aqueous dispersion of polymer particles which will form the particle shell with pre-formed polymer particles which will form the core at a temperature above the glass transition temperature for the shell particles. In order to ensure composite particle formation, certain interfacial energy criteria have to be satisfied for the core particle surface/shell particle surface/liquid interfaces. In order to allow the desired energy relationships to be established, the core particle surfaces include covalently attached ionic groupings which enable a permanent modificato of the core particle surface to be made.

This invention relates to dispersions of composite particles, theparticles being of the type where a first component is partially orwholly engulfed by a second component and the resulting particle is of acolloidal size, and the dispersion being a colloidally stabledispersion. Particles are generally regarded as being of colloidal sizeif they have a particle diameter of less than 10 μm.

Composite particle dispersions of this type are the subject of ourEuropean Publication Number 327,199, corresponding to U.S. Pat. No.4,997,864.

Various processes for the production of composite particles are known inthe art.

In one such process, composite particles are produced by polymerisingmonomers in the presence of core particles. This process involveschemical reactions and does not have control over location of the newpolymer; it does not give complete phase separation if two polymers areused; the core particles are contaminated by reagents involved with thepolymerisation and the core particles must be able to withstand theconditions necessary for the polymerisation of the second polymer. Thereare many examples of this process in the prior art; one example is shownin EP-A-0 195 661 and another example is discussed in a Research Reportby Yi-Cherng Chen entitled "Particle morphology in seeded compositelatexes" published in May 1989 by Lehigh University, Pennsylvania, USA,May 1989.

Processes are also known where composite particles are made and dried toproduce powder. These processes do not result in a colloidally stabledispersion. Examples of such processes appear in US 3 161 602, US 4 800103 and JP-A-80 89855

In another type of process shell material is precipitated onto coreparticles. This process does not give colloidally stable dispersions andinvolves the use of two liquids, one being a solvent and one being anon-solvent. Water cannot be used as the sole liquid for this processSuch solvent liquids are undesirable as they are contaminants of theenvironment and they may give rise to hazards to users. Examples of thistype of process are GB 1 138 570, U.S. Pat. No. 4,016,099 and GB 1 017676.

It is also known to form composite particles where the shell is formedby coacervation, coagulation or complexing processes onto the surface ofcore particles. These processes do not give colloidally stabledispersions; they are also limited to using co-reactive polymer systemsand solvent liquids usually need to be included. Examples of this typeof process are shown in U.S. Pat. No. 4 097 553, U.S. Pat. No. 4 133 774and U.S. Pat. No. 4 440 879.

Our European Publication Number 327,199, corresponding to U.S. Pat. No.4,997,864 describes and claims a process for preparing a dispersion ofcomposite particles, which process comprises mixing first particles witha liquid dispersion of polymer particles, the polymer particles beingstable against particle-particle flocculation and agglomeration, whereinthe mixing takes place at a temperature above the operative glasstransition temperature of the polymer particles and under a conditionwhere ##EQU1## [this relationship will be referred to in this

specification as Relationship I]

where

γ₁₋₃ is the interfacial energy of the first particle surface/aqueousliquid interface

γ₁₋₂ is the interfacial energy of the first particle surface/secondparticle interface

γis the interfacial energy of the second particle surface/aqueous liquidinterface

v_(p) and v_(c) represent the relative volumes of, particle and theaverage first polymer particle, with v_(p) +v_(c=) 1

and where the first particles are able to make contact with the surfacesof the polymer particles so that when contact occurs between firstparticles and polymer particles, composite particles are produced as adispersion in the liquid phase, the particles having stability againstparticle-particle flocculation and agglomeration.

We found difficulties in reliably producing a dispersion by the methodsdescribed in our copending application when the first particles arepolymer particles and are relatively hydrophilic and where the liquid isaqueous. We have however surprisingly found that a substantialimprovement is possible in such cases, especially when the firstparticles are relatively hydrophilic, by including covalently attachedionic groupings on the surface of the first particles.

Accordingly, the present invention provides a process for preparing astable aqueous dispersion of composite particles, which processcomprises mixing an aqueous dispersion of first polymer particles, wherethe particle surfaces include covalently attached ionic groupings togive the particles colloidal stability prior to the mixing, with anaqueous colloidally stable dispersion of second polymer particles, andwherein the mixing takes place at a temperature above the operativeglass transition temperature of the second particles and under acondition where ##EQU2## where γ₁₋₃ is the interfacial energy of thefirst particle surface/aqueous liquid interface

γ₁₋₂ is the interfacial energy of the first particle surface/secondparticle interface

γ₂₋₃ is the interfacial energy of the second particle surface/aqueousliquid interface

v_(p) and v_(c) represent the relative volumes of, respectively, theaverage second polymer particle and the average first polymer particle,with v_(p) +v_(c) =1

and where the first particles are able to make contact with the surfacesof the second particles so that when contact occurs between firstparticles and second particles, composite particles are produced as astable dispersion in the aqueous phase, the composite particles havingcolloidal stability.

Colloidal stability means that collision between particles does not leadto permanent contact between the particles; particle-particleflocculation and agglomeration does not occur and the particles remainfree to move individually within the dispersion.

Methods for measuring the surface energy expression (γ₁₋₃ -γ₁₋₂)/γ₂₋₃and for calculating the expression (1-v_(p) ^(2/3) are described indetail in the European Publication Number 327,199, corresponding to U.S.Pat. No. 4,997,864.

Using this process, it becomes possible to prepare composite particlesas aqueous dispersions, where the core or encapsulated polymer isrelatively hydrophilic such as poly(vinyl acetate), and in a preferredform of the invention, the first particles are more hydrophilic than thesecond particles. Composite particles of this type cannot be prepared byprior art methods which involve polymerisation to form a second polymerin the presence of particles of a first polymer (see for example EP-A-0195 661) or which involve precipitation of one polymer onto particles ofanother polymer.

We believe that our process has the ability to prepare a dispersion ofcomposite particles of this type because the interfacial energies forthe surface of the first particle (to be encapsulated) can be altered inan appropriate way during the process and because the changes to theinterfacial energies are permanent and not merely transitory.

Our process has important advantages over the prior art processes ofwhich we are aware. Our process is essentially a mixing process and nochemical reaction or polymerisation is required. No solvent liquid isrequired; the process can b=operated with water as the sole liquid. Thecomposite particles produced have complete internal phase separation andthe internal boundaries are sharp. The thickness of the encapsulatingshell can be controlled simply by selecting the appropriate size for thefirst and second particles. The first particles, which becomeencapsulated, retain their shape and size in the composite particles andare entirely free from contamination with reagent residues or solventliquids which may be introduced via prior art processes. Our process isthermodynamically-driven and does not require an energy input.

We believe that the mechanism of the process is as follows. The secondparticles retain colloidal stability throughout; ie contacts betweenthese particles do not develop so tht flocculation of these particlesdoes not occur. However, eitehr because the surface of the firstparticles is modified during the process or because the first particlesand second particles are stabilised by a different means, there is notbarrier to contact between a second particle and a first particle; weterm this "heterocontact". Contact between the disimilar particles isfollowed by "engulfment" or spreading of the second polymer oer thefirst particle to produce a "core-shell" structure, because this reducesthe total interfacial energy for the two particles. (The theorysupporting this is detailed in our European Publication Number 327,199,corresponding to U.S. Pat. No. 4,997,864). The resulting compositeparticle has the second polymer particle component at its surface,retains colloidal stability and is stable against flocculation withother composite particles and with second polymer particles.

Where the first particles comprise relatively hydrophilic polymer, withcovalently attached ionic groups, it is usually necessary during theprocess to modify the surface to make it more hydrophobic, that is toincrease the interfacial energy associatd with the core particle-waterinterface (γ₁₋₃) in order to ensure that Relationship (I) is fulfilled.This may conveniently be done by adding a surfactant or polymer carryingan opposite charge to the ionic groups attached at the surface of thefirst particles. If this modification of the particle surface leads tofulfillment of

Relationship (I), engulfment of the first particle by a second polymerwill be thermodynamically driven by a reduction in the total interfacialenergy for the two particles.

However if the ionic groups are not covalently attached at the particlesurface, ie the first particles are not in accordance with theinvention, then addition of a component carrying an opposite charge mayfail to promote engulfment of the first polymer by the second. In thisevent we believe that the relationship (I) is not fulfilled, at least nomore than in a transitory sense, because although the component ofopposite charge combines with the ionic groups at the surface of thefirst particle, the combined materials desorb or otherwise fail toremain as a complete surface layer. This desorption cannot occur if thefirst particle surface has ionic groupings which are covalently attachedto the surface.

The first particles themselves have colloidal stability, and thecomponent added to modify the surface of the first particles may alsoreduce or eliminate the stability of the first particles and can be usedto promote the desired heterocontact with the second particles.

Colloidal stability can be either

(i) ionic, that is generated by ionic groups or surfactants at theparticle surfaces, or

(ii) steric, that is generated by diluent-soluble, oligomeric orpolymeric chains which are adsorbed or linked to the particle surface.

The second polymer particles preferably have steric colloidal stability,that is the surface composition includes oligomer or polymer componentssoluble in the liquid phase. The soluble components at the surface alsoserve to make the particles more hydrophilic (decreasing γ₂₋₃) which isdesirable for fulfillment of the relationship (I). The second polymerparticles may also have surface ionic groups, either anionic orcationic, which will contribute to the colloidal stability of theparticles. These ionic groups may be of opposite or the same charge ason the surface of the first particles.

The first particles are preferably in the form of an aqueous dispersionprior to being mixed with the second particle dispersion.

The polymer particles (either the first or the second particles) mayb=made by emulsion or dispersion polymerisation processes.

Water may be used as the sole liquid in the dispersion of the secondparticles and derived composite particles and also in a dispersion ofthe first particles. Water miscible liquids such as alcohols may beincluded if desired, for example to modify the rate of drying when thedispersion of composite particles produces a film on evaporation.

The polymer particles may include an organic liquid residing at leastpartly inside the polymer particles Such organic liquids may bedesirable either to reduce the operative glass transition temperature ofthe second particles or for example to assist film formation by actingas a plasticiser when the dispersion of composite particles is dried.

The process may be carried out by slowly adding a dispersion of firstparticles to the dispersion of second particles. Components to modifythe surface of the first particles, as solutions or dispersions, may beadded prior to or after the addition of the first particles orpreferably may be added simultaneously and separately when the firstparticles are added to the dispersion of second particles.

A component bearing a charge opposite to the charge responsible forstabilisation of the first particles can be added to the mixture todestabilise the first particles, and this compound can be selected so asto increase the hydrophobic nature of the first particles. The compoundmay be a surfactant, and may be in the form of a polymer.

Alternatively, an electrolyte compound may be added to the mixture todestabilise the charged first particles.

Where an additional component is added to the mixture to destabilise thefirst particles, the further component may be a polymer which includesthe same polymeric components as are present in the second polymerparticles. This, we believe, reduces the interfacial energy at the firstparticle surface/second particle interface (γ₁₋₂) which may be useful inorder to fulfil relationship (I).

The process can conveniently be carried out with a ratio of firstparticles to second particles of approximately 1:1. However a wide rangeof other first particle:second particle ratios have been successfullyused, ranging from about 10:1 to 1:10 and even wider ranges may bepossible.

The respective sizes of the first and second particles may be similar ordifferent. There is no obvious upper limit to the ratio of the diameterof the second particles to the diameter of the first particles, but togive dispersions of particles of colloidal size, the second particlediameter should not exceed approximately 10 μm. However, where thesecond particles are smaller than the first particles, engulfment of thefirst particles by the second may lead to excessive spacing out of thesteric/ionic stabilizer on the surface of the second particles when theparticle spreads around the first particle and in order to maintaincolloidal stability for the composite particles it may be necessaryeither to employ more second particles than first particles or toinclude additional surfactant during the process.

According to a second aspect of the invention, there is provided aprocess for preparing a dispersion of composite particles in an aqueousphase, the composite particles having colloidal stability, which processcomprises mixing an aqueous dispersion of first polymer particles, wherethe particle surfaces include covalently attached ionic groupings togive the particles colloidal stability prior to the mixing, with anaqueous dispersion of second polymer particles where the secondparticles have colloidal stability; wherein the mixing takes place at atemperature above the operative glass transition temperature of thesecond particles and where ##EQU3## where v_(p) and v_(c) represent therelative volumes of, respectively, the average second particle and theaverage first particle, with v_(p+v) _(c) =1 and where θ_(p) is thecontact angle made by the material of the second particles in anenvironment of the liquid phase at a surface which has the surfacecomposition of the first particles.

A method for measuring the contact angle 8, is detailed in our EuropeanPublication Number 327,199, corresponding to U.S. Pat. No. 4,997,864.

The invention extends to stable aqueous dispersions of compositeparticles made by the process set forth above.

According to a third aspect of the invention, there is provided aprocess for making a dispersion of polymer particles of colloidal size,in which process ionic groupings are covalently attached to the particlesurfaces in order to provide sites at which interfacial energy modifyingcomponents can be permanently attached.

The particles, which will form the first particles in the process setforth above for preparation of composite particles, may be prepared inthe presence of ionic surfactant which is copolymerisable because itincludes a double bond which can participate in addition polymerisation.Alternatively the particles may be formed by so called "surfactant-free"methods in which ionic initiators are used and the residues from theinitiation provide ionic colloidal stability for the particles; theseparticles carry disadvantages in that they are very vulnerable toshear-induced flocculation and extra care is required when preparingthem, especially at high concentrations. Ionic monomers such as acrylicacid or methacrylic acid may be used to provide the particles withcovalently attached ionic groups but it is difficult to formulateparticles of this sort to give dispersions with only little coagulation.These particles also are very vulnerable to shear-induced flocculation.It is advantageous to prepare the particles in the presence ofco-polymerisable ionic polymer which is soluble in the aqueous phase.Such ionic polymers may be, produced by reacting polycarboxylic acidwith compounds which include an epoxy ring and a copolymerisable doublebond.

It is especially advantageous to prepare the first particles in thepresence of copolymerisable surface active ionic polymer. Suitablesurface active polymers are graft or block copolymers where one polymeris soluble in the aqueous medium and the other is not, and where one ormore of the polymer components has copolymerisable double bonds.

The invention also extends to colloidal size composite particles made bythe process above. The particles may or may not be in dispersion. Thecomposite particles comprise two or more dissimilar polymers which arephase separated from one another within the particles and have aninternal boundary between the two polymers. The invention furtherextends to composite particles which comprise a relatively hydrophilicpolymer encapsulated by a relatively hydrophobic polymer. By relativelyhydrophilic polymer we mean a polymer which has a lower interfacialenergy when interfacing with water, compared to the other (relativelyhydrophobic) polymer. Where it is inconvenient to measure the relativeinterfacial energies, the relatively hydrophilic polymer may beidentified by reference to published values for the surface tensions ofthe two polymers. The relatively hydrophilic polymer has a surfacetension which is higher and closer to the surface tension of watercompared to the other polymer. Values for surface tensions of polymersare given for example in "Polymer Interface and Adhesion" by Souheng Wu,published by Marcel Dekker Inc. at page 184.

Any polymer may be used to provide first polymer particles (which willbecome the core particles) provided that the polymer is available in orcan be made in particulate form, with covalently attached ionic groupsat the particle surface. The polymers include those derived fromaddition polymerisation of unsaturated monomers.

The polymer may include or comprise, but is not limited to, acrylic acidor any ester such as methyl acrylate, ethyl acrylate, propyl acrylate,butyl acrylate, 2-ethyl hexyl acrylate, glycidyl acrylate; methacrylicacid or any ester such as methyl methacrylate, ethyl methacrylate,propyl methacrylate, butyl methacrylate, lauryl methacrylate, cetylmethacrylate, stearyl I5 methacrylate, ethylene glycol dimethacrylate,tetraethylene glycol dimethacrylate, glycidyl methacrylate,N,N-(methacryloxy hydroxy propyl)-(hydroxyalkyl) amino ethylamidazolidinone; allyl esters such as allyl methacrylate; itaconic acid,crotonic acid or esters; maleic acid or esters such as dibutyl maleate,dioctyl maleate, diethyl maleate; styrene or substituted derivativessuch as ethyl styrene, butyl styrene, divinyl benzene; monomer unitswhich include an amine functionality such as dimethyl amino ethylmethacrylate, butyl amino ethyl methacrylate; monomer units whichinclude an amide functionality such as acrylamide or methacrylamide;vinyl ethers, vinyl thioethers, vinyl alcohol, vinyl ketones, vinylhalides such as vinyl chloride, vinyl fluoride, vinylidene chloride,vinylidene fluoride, tetrafluoroethylene; vinyl esters such as vinylacetate, vinyl versatate; vinyl nitriles, for example acrylonitrile,methacrylonitrile; diene monomer units such as butadiene, isoprene;allyl ethers such as allyl glycidyl ether.

It will be apparent from the description of the process of our inventionthat any polymer may be used to provide second polymer particlesprovided a stable dispersion of the polymer particles is available orcan be made and that the operative glass transition temperature is, oris modified to be, lower than the temperature at which the process is tobe operated. The polymer may include or comprise, but is not limited to,acrylic acid or any ester such as methyl acrylate, ethyl acrylate,propyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate, glycidylacrylate; methacrylic acid or any ester such as methyl methacrylate,ethyl methacrylate, propyl methacrylate, butyl methacrylate, laurylmethacrylate, cetyl methacrylate, stearyl methacrylate, ethylene glycoldimethacrylate, tetraethylene glycol dimethacrylate, glycidylmethacrylate, N,N-(methacryloxy hydroxy propyl)-(hydroxyalkyl) aminoethyl amidazolidinone; allyl esters such as allyl methacrylate; itaconicacid, crotonic acid or esters; maleic acid or esters such as dibutylmaleate, dioctyl maleate, diethyl maleate; styrene or substitutedderivatives such as ethyl styrene, butyl styrene, divinyl benzene;monomer units which include an amine functionality such as dimethylamino ethyl methacrylate, butyl amino ethyl methacrylate; monomer unitswhich include an amide functionality such as acrylamide ormethacrylamide; vinyl ethers, vinyl thioethers, vinyl alcohol, vinylketones, vinyl halides such as vinyl chloride, vinyl fluoride,vinylidene chloride, vinylidene fluoride, tetrafluoroethylene; vinylesters such as vinyl acetate, vinyl versatate; vinyl nitriles, forexample acrylonitrile, methacrylonitrile; diene monomer units such asbutadiene, isoprene; allyl ethers such as allyl glycidyl ether.Alternatively, the second polymer may be a polyoxide such aspoly(ethylene oxide), poly(propylene oxide); a polyester such aspoly(ethylene terephthalate), alkyd; polyurethane; polysulfonate;polysiloxane such as poly(dimethyl siloxane); polysulfide;poly(acetylene); polysulfone; polysulphonamide; polyamide such aspoly(caprolactam), poly(hexamethylene adipamide); polyimine; polyurea;heterocyclic polymer such as polyvinylpyridine, polyvinyl pyrrolidinone;naturally occurring polymer such as natural rubber; gelatin;carbohydrate such as cellulose, alkyl cellulose; polycarbonate;polyanhydride; polyalkene such as ethylene-propylene copolymer.

Ionic polymers suitable for use in the invention may include any of thepolymer components listed for the second particles and may also includeor comprise acidic polymer components such as acrylic acid, methacrylicacid, itaconic acid, crotonic acid and sodium or potassium or othersalts of these, glycidyl methacrylate, N,N -(methacryloxy hydroxypropyl)-(hydroxyalkyl) amino ethyl amidazolidinone, dimethyl amino ethylmethacrylate, butyl amino ethyl methacrylate and other basic monomersand their respective ammonium salts; acrylamide, methacrylamide.

Copolymerisable surfactants and copolymerisable ionic polymers includeat least one double bond which can participate in additionpolymerisation; such double bonds are found for example in acrylates,methacrylates and allyl compounds.

Surface-active polymers include in addition, polymer components whichare insoluble in aqueous media; polymer components of this type arereadily selected by those skilled in the art from the list of polymersgiven above for the second polymer particles.

Dispersions of the polymer particles in an aqueous liquid phase are usedin our process. The particles should not dissolve in the liquid phase.The second particles should retain colloidal stability in the liquidphase. Liquids may be selected from a very wide range. Liquids which areespecially suitable include, without any limitations to these, water,methanol, ethanol, propanol, ethylene glycol, propylene glycol,glycerol, ethoxypropanol, propylene glycol methyl ether, ethoxyethanol,butoxyethanol.

The dispersion of second polymer particles is preferably a stable latexwhich may be synthetic or naturally occurring.

EXAMPLE 1

A copolymerisable, ionic, surface-active polymer was prepared asfollows:

    ______________________________________                                        STAGE 1           Parts by weight (gms)                                       ______________________________________                                        A     ethanol         1358                                                          azodiisobutyronitrile                                                                         3.4                                                           (ADIB)                                                                  B     acrylic acid    1125                                                          ADIB            11.2                                                          primary octyl mercaptan                                                                       5.6                                                           (POM)                                                                   C     ADIB            1                                                       ______________________________________                                    

A glass, round bottom reaction vessel (5 liter capacity) was fitted withlid, condenser, stirrer, thermometer and nitrogen supply.

Charge A was placed in the vessel and the temperature raised to refluxand held for 30 minutes. The reflux was maintained and mix B was addedat a steady rate over 2 hours. Reflux was continued for a further 30minutes before adding C and then refluxing for a further 45 minutes. Theproduct which was a homopolymer polyacrylic acid of relatively lowmolecular weight was allowed to cool and to stand in contact with air.

The Stage I intermediate product was a clear, viscous solution with apolymer content of 46.2%.

    ______________________________________                                        STAGE II           Parts by weight                                            ______________________________________                                        Stage I product    2507                                                       (solution of polyacrylic acid)                                                glycidyl methacrylate                                                                            22.5                                                       Armeen DMCD*       15.1                                                       ______________________________________                                    

The mix was raised to reflux, held for 5 hours and then allowed to cool.

During this stage, the epoxy rings of the glycidyl methacrylate reactwith some of the carboxylic acid groups to leave pendant double bondswhich are available to prepare block or graft copolymers in Stage III.

The Stage II intermediate product was a yellow, viscous solution with apolymer content of 46.7%.

    ______________________________________                                        STAGE III        Parts by weight                                              ______________________________________                                        A       ethanol      400                                                              ADIB         0.2                                                      B       ethyl acrylate                                                                             335                                                              ADIB         3.8                                                              POM          16.3                                                             Stage II product                                                                           806                                                              ethanol      200                                                      C       ADIB         0.4                                                      ______________________________________                                    

Charge A was raised to reflux and held for 30 minutes With the refluxmaintained, mix B was added at a steady rate over 21/2 hours. Reflux wascontinued for a further 1 hour, C was added and the reflux continued fora further 1 hour. The product was allowed to cool and to stand in air.

During Stage III, the ethyl acrylate polymerised and formed anill-defined block or graft copolymer with most of the pendant doublebonds from Stage II consumed. The copolymer still possessed reactivecarboxylic groups from the polyacrylic acid.

The Stage III intermediate product was a viscous solution with no gelcontent and with a polymer content of 40.1%.

    ______________________________________                                        STAGE IV        Parts by weight                                               ______________________________________                                        Stage III product                                                                             1762                                                          glycidyl methacrylate                                                                         7.2                                                           Armeen DMCD*    5.4                                                           ______________________________________                                    

The temperature was raised to reflux and held for 7 hours. In Stage IV,more epoxy rings are available to react with some carboxylic groups toleave pendant double bonds. These double bonds are then the points bywhich the Stage IV product, which is a surfactant, becomes covalentlyattached to the first polymer particles.

The product was a viscous solution with no gel content and with apolymer content of 40.5%.

*Armeen DMCD is a Trade Mark of the Akzo company. The compound is acatalyst for the ring opening reaction between the carboxylic acid andthe epoxy compound.

EXAMPLE 2

A dispersion of ionically-stabilised poly(vinyl acetate) particles wasprepared using the copolymerisable, ionic, surface-active polymer madein Example 1.

The apparatus comprised a glass culture vessel with parallel sides (2liter capacity), fitted with lid, turbine stirrer, thermometer, nitrogensupply and condenser. The glass vessel was supported with the lower onethird immersed in a hot-water bath.

    ______________________________________                                                          Parts by weight                                             ______________________________________                                        A     water (demineralised)                                                                           687                                                         Example I product 60                                                          1 molar alcoholic potassium                                                                     98.3                                                        hydroxide                                                               B     vinyl acetate     28.4                                                  C     ammonium persulphate                                                                            0.6                                                         water             11.8                                                  D     vinyl acetate     534.1                                                       ammonium persulphate                                                                            1.8                                                         water             35.7                                                  ______________________________________                                    

Charge A was raised to 65° C. Compound B was added. After 10 minutes Cwas added and the temperature held for 45 minutes after which time, thecontent comprised a dispersion of "seed" polymer particles. Addition ofD and E was started and these were added separately and simultaneouslyusing two peristaltic pumps at a steady rate over a period of 21/2hours. The temperature was maintained for a further 45 minutes and thedispersion was then allowed to cool.

The product was a colloidally stable dispersion in water of PVAparticles (first particles) which were anionically stabilised. Thepolymer content was 39.2%. The mean particle diameter was 515 nm asdetermined using a Coulter Nanosizer particle-sizing-instrument. Thedispersion did not contain any significant amount of coagulum and therewere no polymer deposits on the walls of the reaction vessel or on thestirrer

EXAMPLE 3

A dispersion of ionically-stabilised, internally cross-linked poly(vinylacetate) particles was prepared, by including a difunctional monomer inthe formulation to produce internal cross-linking in the particles.

The difunctional monomer was tetraethylene glycol dimethacrylate.

    ______________________________________                                                          Parts by weight                                             ______________________________________                                        A     water (demineralised)                                                                           687                                                         Example 1 product 60                                                          1 molar alcoholic potassium                                                                     98.3                                                        hydroxide                                                               B     vinyl acetate     27.8                                                        tetraethylene glycol                                                                            0.57                                                        dimethacrylate                                                          C     ammonium persulphate                                                                            0.59                                                        water             11.8                                                  D     vinyl acetate     523                                                         tetraethylene glycol                                                                            10.7                                                        dimethacrylate                                                          E     ammonium persulphate                                                                            1.79                                                        water             35.7                                                  ______________________________________                                    

The apparatus and process was the same as used in Example 2.

The product was a stable dispersion of internally cross-linked PVAparticles (first particles) which were anionically stabilised againstflocculation and aggregation. The polymer content was 39.1%. The meanparticle diameter was 662 nm as measured using a Coulter Nanosizer.

EXAMPLE 4

A dispersion of ionically-stabilised poly(vinyl acetate) particles wasprepared using copolymerisable ionic polymer (polyacrylic acid withpendant double bonds) as made at Stage II in Example 1.

    ______________________________________                                                          Parts by weight                                             ______________________________________                                        A     water             800                                                         Example 1, Stage II                                                                             20                                                          intermediate                                                                  1 molar alcoholic potassium                                                                     49.1                                                        hydroxide                                                                     amonium persulphate                                                                             1.0                                                   B     vinyl acetate     200                                                   ______________________________________                                    

The apparatus used was as in Example 2. Components A were charged to thereactor and stirred and B was added. The mix was purged with nitrogenfor 1 hour. The temperature was raised to 65° C. and held for 2 hours.

The product was a dispersion of ionically-stabilised particles with amean size of approximately 1000 nm. The polymer content was 35.5%.

EXAMPLE 5

Composite particles were made by mixing a dispersion of first polymerparticles, comprising poly(vinyl acetate) with a dispersion of secondpolymer particles comprising vinyl acetate/vinyl versatate copolymer, inthe presence of a further component which reduced the stability of thefirst particles and altered the interfacial energies.

The dispersion of first particles was prepared as in Example 2 using thecopolymerisable, ionic, surface-active polymer as made in Example I. Theparticles were anionically stabilised against flocculation andaggregation. The mean particle diameter was I5 nm and the polymercontent of the dispersion was 39.2%.

The dispersion of second particles was prepared by polymerising a 80/20(w/w) mixture of vinyl acetate and vinyl versatate in water in thepresence of methoxy poly (ethylene oxide) methacrylate of approximately2000 molecular weight. The particles were largely sterically stabilisedby the soluble poly(ethylene oxide) chains which were covalentlyattached to the surfaces of the particles and partly stabilised byanionic groups which arose from the ionic initiator which had been usedto generate polymerisation. The mean particle diameter was 650 nm andthe polymer content of the dispersion was 41.9%.

    ______________________________________                                                          Parts by weight                                             ______________________________________                                        A     dispersion of first particles                                                                   20.0                                                        (from Example 2)                                                        B     dispersion of second                                                                            35.6                                                        particles                                                               C     0.1 molar solution of                                                                           23.4                                                        dodecyl trimethyl ammonium                                                    bromide                                                                       (DOTAB)                                                                 ______________________________________                                    

The ratio of first particles:second particles was approximately 1:1 bynumber.

Dispersion B was charged to a beaker and maintained at 25° C. withstirring throughout the process. Dispersion A and component C were addedseparately but simultaneously using syringe pumps; they were added at asteady rate over 11/2 hours.

This product was a colloidally stable dispersion of composite particleswith a small number of particle floccs present. It was shown by testswhich are described below under the heading Test Method that thecomposite particles were sterically stabilised and thationically-stabilised particles were not present in significant numbers.This indicates that the ionically-stabilised first particles had beenengulfed by the sterically-stabilised second particles, and as a resultthat composite particles had been formed.

EXAMPLE 6

In this comparative Example, Example 5 was repeated except thatcomponent C was omitted.

The product was a stable dispersion. However the tests showed that bothionically-stabilised and sterically-stabilised particles remained andthis indicates that the first particles with covalently attached ionicgroupings at the surface had not been encapsulated by the secondparticles.

EXAMPLE 7

Composite particles were made where the first particles comprisedinternally cross-linked poly(vinyl acetate) as prepared in Example 3.

The second particles were as used in Example 5. A random copolymer ofvinyl acetate/vinyl versatate/dimethyl amino ethyl methacrylate (55/40/5w/w) was used as the component (C) in place of DOTAB to modify surfaceenergies and stability of the first particles.

    ______________________________________                                                          Parts by weight                                             ______________________________________                                        A     dispersion of first                                                                             23.6                                                        particles (as made in                                                         Example 3)                                                              B     dispersion of second                                                                            20.0                                                        particles                                                               C     amino-contianing random                                                                         19.7                                                        copolymer as 45.5% solution                                                   in ethanol                                                              ______________________________________                                    

The ratio of first particles:second particles ≈1:1 by number. The methodused was as in Example 5.

The product was a colloidally stable dispersion with a small number ofparticle floccs. The tests showed that the particles in the dispersionwere sterically-stabilised and that ionically-stabilised particles werenot present in significant numbers.

TEST METHODS used in Example 5, 6 and 7

Two tests were used to determine whether particular colloidally stableparticle dispersions were sterically stabilised or ionically-stabilised.

TEST 1

A small sample of dispersion was placed between the cone and plateplatens of a viscometer and subjected to a shear rate of 10,000 sec⁻¹for 90 seconds. The apparent viscosity was monitored during this time.Generally, sterically-stabilised particle dispersions retain stableunder this test and show only a small increase in apparent viscositywhereas ionically-stabilised particles flocculate and give a largeincrease in apparent viscosity.

TEST 2

A sample of dispersion was mixed with an equal volume of a 10% solutionof calcium chloride in water. Generally, sterically-stabilised particledispersions remain stable under this test whereas ionically-stabilisedparticles flocculate.

TEST RESULTS

The dispersions of first particles, the dispersions of second particlesand the product dispersions from Examples 5, 6 and 7 were subjected tothese tests.

    ______________________________________                                                           Test 1  Test 2                                             ______________________________________                                        Example 5  First particles                                                                             x         x                                                     Second particles                                                              Final particles                                                                             slight flocc.                                        Example 6  First particles                                                                             x         x                                                     Second particles                                                              Final particles                                                                             x         x                                          Example 7  First particles                                                                             x         x                                                     Second particles                                                              Final particles                                                    ______________________________________                                           remained stable                                                             x  flocculated                                                           

These results indicate that encapsulation of the first particles by thesecond particles (which were primarily sterically-stabilised) waslargely achieved in Examples 5 and 7 but not in comparative Example 6.

EXAMPLE 8

Composite particles were made by mixing a dispersion of poly(vinylacetate) particles which were stabilised with copolymerisable, anionicpolymer with a dispersion of acrylic particles in the presence of acationic surfactant to reduce stability and alter the interfacialenergies.

A dispersion of first particles (PVA) was prepared by a process similarto Example 4 except that the amount of copolymerisable, ionic polymerwas increased. The mean particle diameter was 470 nm and the polymercontent of the dispersion was 37.9%.

A dispersion of second particles had been prepared by polymerisingmethyl methacrylate/2-ethyl hexyl acrylate/acrylic acid (51/49/1 w/w) inan aqueous solution of a poly ethoxylate non-ionic surfactant and sodiumcarboxy methyl cellulose as protective colloid. The mean particle sizewas 660 nm and the polymer content of the dispersion was 58.9%.

    ______________________________________                                                           Parts by weight                                            ______________________________________                                        A     dispersion of first particles                                                                    20.0                                                 B     dispersion of second particles                                                                   34.0                                                 C     0.1 molar solution of dodecyl                                                                    7.2                                                        trimethyl ammonium bromide                                              ______________________________________                                    

The ratio of first particles:second particles was approximately 1:1 bynumber.

Dispersion B was charged to a beaker and stirred at 25° C. Dispersion Aand component C were added separately and simultaneously to the beakerat a constant rate over 11/2 hours.

The product was a colloidally stable dispersion of composite particleswith a small number of aggregates present. Characterization using a disccentrifuge indicated that the final dispersion comprised particles whichwere slightly larger than the second particles.

EXAMPLE 9

Composite particles were made by mixing a dispersion of first polymerparticles comprising poly(butyl acrylate) with a dispersion of secondparticles comprising poly(ethyl acrylate). Prior to the process thefirst particles had ionic colloidal stability arising from ionic groupswhich were residual from the ionic initiator and which resided at theend of the particle polymer chains and which therefore were covalentlyattached. A further component was added during the process to modify thesurface of the first particles.

The dispersion of first particles was prepared by polymerising butylacrylate in water with ammonium persulphate as initiator and in theabsence of any surfactant or preformed polymer. The mean particlediameter was 220 nm and the polymer content of the dispersion was 3%.The dispersion was colloidally stable.

The dispersion of second particles was prepared by polymerising ethylacrylate in a dilute solution of an amphipathic graft copolymer inwater, using a non-ionic initiator (azo-diisobutyrnitrile). The graftcopolymer comprised 50/45/5 methoxy poly(ethyleneoxide) methacrylate(M.W=2000)/butyl acrylate/ dimethylaminoethyl methacrylate; this hadpreviously been prepared by polymerisation in ethanol to give a 45%solution/dispersion in ethanol. The ratio of graft copolymer topoly(ethyl acrylate) in the dispersion was 3.8:100. The particles hadsteric colloidal stability. The mean particle diameter was 250 nm andthe polymer content of the dispersion was 3%.

    ______________________________________                                                            Parts by weight                                           ______________________________________                                        A      dispersion of first particles                                                                    7.1                                                 B      dispersion of second particles                                                                   10.0                                                C      0.01 molar solution of DOTAB                                                                     1.6                                                 ______________________________________                                    

The ratio of first particles to second particles was approximately 1:1by number. The process was operated as in Example 5. The product was acolloidally stable dispersion of composite particles with a meandiameter of 340 nm.

EXAMPLE 10

This was similar to Example 9 except that the first particles comprisedpoly(vinyl acetate).

The first particles were stabilised by covalently attached ionicinitiator residues as in Example 9. The mean particle diameter was 230nm and the polymer content of the dispersion was 3%.

The second particles were as in Example 9. The mean particle diameterwas 250 nm and the polymer content of the dispersion was 3%.

    ______________________________________                                                            Parts by Weight                                           ______________________________________                                        A      dispersion of first particles                                                                    8.2                                                 B      dispersion of second particles                                                                   10.0                                                C      0.01 molar solution of DOTAB                                                                     1.5                                                 ______________________________________                                    

The ratio of first particles to second particles was approximately 1 to1 by number.

The product was a colloidally stable dispersion of composite particleswith a mean size of 290 nm.

EXAMPLE 11

Composite particles were made where the first particles comprisedpoly(ethyl acrylate) and were stabilised by covalently attached ionicgroups. The second particles comprised poly(butyl acrylate); that is thepolymer of the first particles was relatively hydrophilic (surfacetension=37.0 dyne.cm⁻¹ at 20° C.) compared to the second polymer(surface tension=33.7 dyne.cm⁻¹ at 20° C.).

The first particles were prepared by polymerising ethyl acrylate inwater without surfactant, using a high level of ammonium persulphate asinitiator (25 parts of initiator to 100 parts of the monomer). The meanparticle size was 170 nm and the polymer content of the dispersion was3%.

The second particles were prepared using a solution of the amphipathicgraft copolymer as in Example 9. The ratio of graft copolymer topoly(butyl acrylate) in the dispersion was 4.7:100. The mean particlediameter was 190 nm and the polymer content of the dispersion was 3%.

    ______________________________________                                                            Parts by Weight                                           ______________________________________                                        A      dispersion of first particles                                                                    7.1                                                 B      dispersion of second particles                                                                   10.0                                                C      0.01 molar solution of DOTAB                                                                     1.6                                                 ______________________________________                                    

The ratio of first particles to second particles was approximately 1:1by number.

The process was operated as in Example 5 with very gentle stirring. Theproduct was a colloidally stable dispersion of composite particles witha very small proportion of the polymer present as aggregates. It isconsidered that these aggregates were produced as a result ofshear-induced flocculation of a small proportion of the core particlesduring the stirring.

EXAMPLE 12

In this comparative example, Example 11 was repeated except thatcomponent C (which modified the surface of the first particles) wasomitted. The product was a dispersion with some flocculated aggregates.A portion of the product was decanted into a glass vial and capped. Asample of similar size was taken from the product of Example 11. The twocapped glass vials were shaken vigorously by hand for one minute. Thedispersion from Example 11 remained stable and no flocculation oraggregation was evident. However the dispersion from this comparativeexample gave a considerable amount of flocculation and aggregation,indicating that the core particles (which flocculate on vigorousshaking) had not been encapsulated by the second particles (which do notflocculate on shaking).

EXAMPLE 13

Comparative experiments were conducted in which the ionic groups at thesurface of the first particles were not covalently attached at theparticle surface but were able to desorb if this was favoured whenconditions changed. Poly(ethyl acrylate) (PEA) (relatively hydrophilic)and poly(butyl acrylate) (PBA) were used as polymers.

The first polymers were prepared by polymerisation in water using anon-ionic initiator (azodiisobutyronitrile) and a conventional anionicsurfactant (dioctyl sodium sulpho succinate) which was free to adsorb ordesorb from the particle surface.

The second particles were prepared using amphipathic graft copolymer andnon-ionic intiator as in Examples 9 and 11.

In each experiment, the ratio of first particles : second particles wasapproximately 1:1 by number and the process was operated as in Example 5except that in two experiments component C (DOTAB solution) was omitted.

The results are set out in the following Table:

                                      TABLE                                       __________________________________________________________________________         First                                                                              Second                                                                             Component                                                           Particle                                                                           Particle                                                                           C                                                              Expt No.                                                                           Polymer                                                                            Polymer                                                                            (DOTAB soln)                                                                          Observations    Result                                 __________________________________________________________________________    1    PEA  PBA  present Progressive flocculation                                                                      FAILED                                      D = 210                                                                            D = 270      throughout process                                     2    PEA  PBA  absent  Gross flocculation developed                                                                  FAILED                                      D = 210                                                                            D = 270      within 1 hour after process                            3    PBA  PEA  present No flocculation was observed.                                                                 SUCCESSFUL                                  D = 190                                                                            D = 250      Product was colloidally stable                                                dispersion of composite particles.                     __________________________________________________________________________    4    PBA  PEA  absent  No flocculation was observed.                                                                 SUCCESSFUL                                  D = 190                                                                            D = 250      Product was colloidally stable                                                dispersion of composite particles.                     __________________________________________________________________________     D is mean diameter in nm as dertermined using a Coulter Nanosizewr            particle sizing instrument.                                              

With those systems in which ionic groups were not covalently attached atthe surface of the first particles, the process was unsuccessful whenthe first particles comprised relatively hydrophilic polymer but wassuccessful when the first polymer was relatively hydrophobic.

We claim:
 1. A process for preparing a stable aqueous dispersion ofcomposite particles, which process comprises mixing an aqueousdispersion of first polymer particles, where the particle surfacesinclude covalently attached ionic groupings to give the particlescolloidal stability prior to the mixing, with an aqueous colloidallystable dispersion of second polymer particles, and wherein the mixingtakes place at a temperature above the operative glass transitiontemperature of the second polymer particles and under a condition where##EQU4## Where γ₁₋₃ is the interfacial energy of-the first polymerparticle surface/aqueous liquid interfaceγ₁₋₂ is the interfacial energyof the first polymer particle surface/second polymer particle interfaceγ₂₋₃ is the interfacial energy of the second particle surface/aqueousliquid interface V_(p) and V_(c) represent the relative volumes of,respectively, the average second polymer particle and the average firstpolymer particle, with V_(p) +V_(c) =1and where the first polymerparticles are able to make contact with the surfaces of the secondpolymer particles so that when contact occurs between first polymerparticles and second polymer particles, composite particles are producedas a stable dispersion in the aqueous phase, the composite particleshaving colloidal stability.
 2. A process according to claim 1 , whereinthe first polymer particles are more hydrophilic than the second polymerparticles.
 3. A process according to claim 1, wherein a surfactant isattached to or formed on the ionic groupings to produce the desiredinterfacial energy conditions at the interfaces.
 4. A process accordingto claim 1, wherein the second polymer particles have a surfacecomposition which includes oligomer or polymer components soluble in theliquid phase sufficient to give the second polymer particles ahydrophibic surface.
 5. A process according to claim 1, wherein thefirst polymer particles or the second polymer particles or both are madeby an emulsion polymerisation process.
 6. A process according to claim1, wherein the second polymer particles include an organic liquidresiding at least partly inside the second polymer particles.
 7. Aprocess according to claim 1, in which the dispersion of first polymerparticles is added slowly to the dispersion of second polymer particles.8. A process according to claim 1, wherein the second polymer particlesare at least partly sterically stabilised.
 9. A process according toclaim 1, wherein a compound of charge opposite to the charge responsiblefor stabilisation of the first polymer particles can be added to themixture to destabilise the first polymer particles, and this compoundcan be selected so as to increase the hydrophobic nature of the firstpolymer particles.
 10. A process according to claim 9, wherein thecompound of charge opposite to the charge responsible for stabilisationof the first polymer particles is a surfactant, and is in the form of apolymer.
 11. A process according to claim 9, wherein an electrolytecompound is added to the mixture to destabilise the charged firstpolymer particles.
 12. A process according to claim 1, wherein a furthercomponent is added to the mixture of destabilise the mixture, whichfurther component is a polymer which has monomer units which are commonto both the further component and to the second polymer particles.
 13. Aprocess according to claim 1, wherein the process is carried out with aratio of first polymer particles to second polymer particles ofapproximately 1:1.